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AI'l'KNDIX                                     287

If the para-position is already occupied, the nitrogen  takes
the ortho-position to the amino-group,

but the reaction only takes place readily where the para-position is
free. The manner in which the change is brought about has not
been satisfactorily explained, although from the fact that/-cliazo-
aminotoluene yields, on warming with aniline hydrochloride,
^-toluene a/.oaminoben/ene and /Moluidine,

CII;,.(        ' N: N. N11/        \C 113 + '('        )N IL,

:      HI.,-          ,N:N/        X;Nil,, -I CII,/        "/Nil,,,

\ ....._/         \__/        ~         \__/

it would appear as if the hydrochloride of the base were the
chief factor in the decomposition, and that the change was
rather inter- than ////nr-molecular. Aminoaxoben/ene, under
the name ol' tiirilincyclloiv, has been used as a colouring matter.
Its chief technical application at present is in the manufacture
of a class of dark blue colours, known as hitfttlhies. On reduc-
tion with tin and hydrochloric acid, it decomposes into two
moltM ules of base, aniline and />-phenylenediamine, a reaction
which is shared by most of the a/.o-compounds (see p. 176),

Phenylhydrassine, Phenylmethylpyrazolone.  The
use of phcnylhydra/,UK* or, in some cases /;~bromo~ or /-nitro-
plurnylhydra/.inc, as a reagent for the detection of aldehydes and
ketones, has been illustrated in the reactions on p. 70.
One of its most important technical uses is in the prepara-
tion of antipyriw, in which the product, obtained by the action